The invention relates to a variable magnification slitwise exposure process and an exposure system therefor which may be used in electrophotographic copying machines.
It would be convenient if a single electrophotographic copying machine could be utilized to provide a copied image of varying magnifications selectively while using an original of a given size. To this end, a variety of exposure techniques have been proposed to provide a variable magnification.
Referring to FIG. 1, there is shown one example of an optical exposure system, for carrying out a variable magnification slitwise exposure process, which has been heretofore proposed. The system comprises a lens L and a reflecting mirror system 2 to form an optical path between an original 0 and a photosensitive member 1 so as to focus a real image of the original onto the surface of the photosensitive member 1 through the lens L. Section I of FIG. 1 represents the arrangement of the reflecting mirror system 2 and the lens L for a magnification of unity. At this time, the original is moved in the direction indicated by an arrow at a rate of V.sub.0. A portion of the original which passes through a point p.sub.0 is focussed onto the surface of the photosensitive member 1 which passes through a point q.sub.0 which represents a fixed point in space of the system. It will be understood that the original must be placed in a manner such that its leading edge, as viewed in the direction of its movement, is located at the point p.sub.0 at the commencement of a copying operation.
If a copy image of a different magnification is desired, the reflecting mirror system 2 and the lens L must be transposed to different positions and the rate of movement of the original changed from V.sub.0 to V.sub.1 depending on the new magnification, as illustrated in Section II of FIG. 1. This changes the length of the optical path from the original to the photosensitive member 1, so that, if the focussing point q.sub.0 is to be maintained stationary, the leading end of the original must be transposed to a different point p.sub.1 at the commencement of a copying operation. Thus, the position in which the original is placed must be changed depending on the magnitude of the magnification desired, which is inconvenient for practical purposes.
While it is possible to maintain the same position of the original and the same rate of movement of the original when the magnification is varied while employing an optical exposure system as illustrated, it is then necessary to change the position and the rate of rotation of the photosensitive member 1 in accordance with the magnitude of the magnification, which results in a complex construction of the copying machine. In addition, since the rotation of the photosensitive member 1 governs the entire copying process, the timing of operating various units associated with the charging, exposure and developing steps will have to be changed as the rate of rotation of the photosensitive member is changed in accordance with the magnification, resulting in a further inconvenience. Such a difficulty can be avoided in a so-called flash exposure system in which both the original and the photosensitive member remain stationary. However, this technique involves an increase in the overall size even for a magnification of unity. If such system is constructed for a variable magnification, the exposure area of the photosensitive member must be increased by a factor corresponding to the magnification which is intended to be employed during the copying process, thus further increasing the overall size of the system.